Hardiboard is tough board that won't be bothered by water for quite a while. Metal is better if protected from rust because hardiboard can still be broken. The cost is not that much different, but the board is easier to work with so its almost a tie.
Hardiboard skirting is more attractive but does not have the durability of metal skirting. Hardiboard tends to buckle over time and is apt to seperate at the seams.